The present invention relates to an apparatus for injection molding plastic articles and for an improved venting arrangement for use therein.
Various types of elongated, thin-walled, molded plastic articles are presently manufactured and used for many different purposes. Such articles include containers, open-ended pails, parts for ball-point pens, probe cover devices, and sheaths. A number of different molding apparatuses are used to manufacture these articles.
U.S. Pat. Nos. 3,660,002 to Morroni; 3,978,186 to Lovejoy; 4,381,275 to Sorensen; 4,622,002 to Bormuth; 4,726,925 to Binder; 4,743,420 to Dutt; and 4,764,103 to Mitake and published European Patent Application No. 0 124 951 to van Noort illustrate some of the apparatuses used to manufacture the aforementioned articles. Generally, each of the molding systems illustrated in these patents includes a mold having a first part defining a mold cavity and a second part defining a mold core. Each of the mold cores and respective mold cavities when in a mold closed position define a mold space in the shape of the article to be manufactured. In some systems, such as van Noort's, the mold cavity is defined by two parts which move relative to each other to facilitate removal of the molded article.
The plastic material used to form the molded article can be injected into the mold space in a variety of different ways. As shown in the Dutt and Mitake patents, it is known to inject the molten plastic material through a central gate adjacent the portion of the mold space forming the bottom of the molded article. This type of gating arrangement has traditionally been used in many injection molding machines. In other systems, such as that shown in the Sorenson patent, molten plastic material is injected into the mold space through gates positioned along sidewall defining portions of the mold space. Still further, it is known to inject molten plastic material into that portion of the mold space defining a lip of the article. The Bormuth patent illustrates one such system.
Another patent which illustrates a system for injecting molten plastic material into the lip portion of the space defining the article to be molded is U.S. Pat. No. 5,145,630 to Schad. In the Schad system, the molten plastic material is injected into the mold space at the lip portion using multiple injection gates in a manner which does not sacrifice core/cavity alignment. In other words, the molten material is injected into the mold space in a manner which substantially avoids shifting of the mold core.
Still another approach for injecting molten plastic material into a mold space defining a container is illustrated in U.S. Pat. No. 4,277,435 to Allen. In the Allen system, the plastic material is injected into the mold space near the lip defining portion of the mold space. It is injected somewhat tangentially to cause the plastic material to move peripherally in the rim defining portion of the mold space. This is done in order to have the weaker direction strengths also extend peripherally and thereby increase the container's resistance to lengthwise splitting from the rim to the bottom. The system used by Allen vents the mold adjacent the bottom defining portion of the mold space.
Mold venting is significant in that every mold contains air that must be removed or displaced as the mold space is being filled with plastic material. At high speed injection, insufficient mold venting may produce a considerable compression of the air, with consequent slow or incomplete mold filling, premature plastic pressure buildups, and, in extreme cases, burning of the plastic. The Injection Molding Handbook edited by Dominick V. Rosato et al., published by Van Nostrand Reinhold Company, discusses the significance of mold venting on pages 186-189.
U.S. Pat. No. 4,126,291 to Gilbert et al. illustrates an apparatus for molding thermometer probe covers having a bottom venting arrangement. The apparatus includes a tungsten carbide core which minimizes core deflection to maintain improved concentricity of core and cavity. Maintenance of concentricity is improved by a combination of a relatively loose cantilevered mounting of an end of the core together with a press fit locking of the core upon the closing of the mold. Venting of cavity gas during injection of the mold material is controlled through symmetrically distributed vent orifices in both the core and the female mold parts. Core venting is controlled by a plurality of orifices and a manifold provided by a water cooled core vent pin within a bore of the core. This venting feature is shown in FIGS. 8, 9, 10, 11, and 16.
When making plastic articles using rim gating techniques, such as those shown in the Schad patent, there have been problems in obtaining proper venting of the mold space. In some situations where there was a core shift, a large burn mark would appear on the bottom of the part where air had been trapped due to inadequate or inefficient venting. In a production environment, it is highly desirable to have a venting arrangement that substantially avoids the problem of burning and significantly reduces the sensitivity of the molding apparatus to core shift.
Accordingly, it is an object of the present invention to provide a molding apparatus with an improved venting arrangement.
It is a further object of the present invention to provide a molding apparatus as above with a venting arrangement that helps to substantially avoid problems such as burning and reduces the sensitivity of the mold apparatus to core shift.
It is still another object of the present invention to provide a molding apparatus as above with an improved venting arrangement that can be used with a rim gate injection system.
Still other objects and advantages of the present invention will become more apparent from the following description and the accompanying drawings in which like reference numerals depict like elements.